Horizontal tail load optimization system and method

ABSTRACT

A method of controlling an elevator of an aircraft includes selecting a factor for increasing or decreasing a predetermined horizontal tail load alleviation (HTLA) authority limit for an elevator based on at least one aircraft parameter. The HTLA authority limit decreases with an increase in Mach number and/or airspeed. The method also includes computing an elevator position limit as a product of the HTLA authority limit and the factor, and moving the elevator to a commanded elevator position that is no greater than the elevator position limit.

FIELD

The present application is a divisional of and claims priority topending application Ser. No. 14/278,868, filed May 15, 2014, entitledSYSTEM AND METHOD FOR OPTIMIZING HORIZONTAL TAIL LOADS, the entirecontents of which is incorporated herein by reference.

FIELD

The present disclosure relates generally to flight controls and, moreparticularly, to systems and methods of optimizing tail loads on anaircraft.

BACKGROUND

Pitch control of an aircraft is the control of the nose-up and nose-downpitch attitude of the aircraft during flight. During changes in pitchattitude, the aircraft pivots about a lateral axis extending through acenter of gravity (CG) of the aircraft. Pitch control may be provided bya horizontal tail that may be located aft of the aircraft wings. Thehorizontal tail may include an adjustable horizontal stabilizer and anelevator. The elevator may be pivotably coupled to the horizontalstabilizer. The angle of incidence of the horizontal stabilizer may beadjusted in a positive and/or negative direction relative to alongitudinal axis of the aircraft to trim the aircraft so that theaircraft is maintained at a constant pitch angle during flight. Forexample, the horizontal stabilizer may be adjusted at a negative angleof incidence to provide a download so that a nose-up pitching moment isgenerated about the aircraft CG to counteract a nose-down pitchingmoment generated by the positive lift of the wings. The load generatedby the horizontal tail may be carried by the structure of the horizontaltail and the fuselage.

For nose-up and nose-down pitch maneuvering of the aircraft duringflight, the elevator may be pivotably moved upwardly or downwardly froma neutral position relative to the horizontal stabilizer. For example,to maneuver the aircraft from a nose-up attitude to a level attitude,the elevator may be pivotably deflected or positioned downwardly (e.g.,elevator trailing edge down) so that the horizontal tail generates anincreased amount of upload to bring the nose of the aircraft down. Theelevator may be held in the downward position until a level attitude ofthe aircraft is achieved, after which the elevator may be returned tothe neutral position. To maneuver the aircraft from a nose-down attitudeto a level attitude, the elevator may be pivotably deflected orpositioned upwardly (e.g., elevator trailing edge up) so that thehorizontal tail generates an increased amount of download to bring thenose of the aircraft up until a level attitude of the aircraft isachieved.

As the speed of an aircraft increases, dynamic pressure on thehorizontal stabilizer and elevator also increases which results in anincrease in the upload or download generated by the horizontal tail. Toavoid exceeding the load-carrying capability of the horizontal tail andfuselage during nose-up or nose-down maneuvering, the movement orauthority of the elevator may be electronically limited as speedincreases. Limiting the elevator authority may also provide a morelinear or uniform pitch response from pilot input as a function ofairspeed. In addition, limiting the elevator authority may preventexcessive maneuvering capability at relatively high dynamic pressure.Unfortunately, excessively limiting the elevator authority as a functionof airspeed to reduce tail loads may result in sub-optimal maneuveringcapability.

As can be seen, there exists a need in the art for a system and methodfor moving the elevator within limits that reduce tail loads whileproviding sufficient authority for pitch control of the aircraft.

SUMMARY

The above-noted needs associated with elevator control are specificallyaddressed by the present disclosure which provides a method ofcontrolling an elevator of an aircraft. The method may includeidentifying a current stabilizer angle of incidence of a stabilizer ofthe aircraft. The stabilizer may include an elevator pivotably coupledto the stabilizer. The method may further include comparing the currentstabilizer angle of incidence with a threshold stabilizer angle ofincidence, and selecting an elevator position limit that is morerestrictive if the current stabilizer angle of incidence is greater thanor equal to the threshold stabilizer angle of incidence. The method mayadditionally include moving the elevator to a commanded elevatorposition that is no greater than the elevator position limit.

Also disclosed is a system for controlling an elevator of an aircraft.The system may include a flight control processor configured to receivea stabilizer signal representative of a current stabilizer angle ofincidence of a stabilizer. As indicated above, the stabilizer mayinclude an elevator that maybe pivotably coupled to the stabilizer. Theflight control processor may be configured to select an elevatorposition limit based on the stabilizer signal. The selected elevatorposition limit may be more restrictive if the current stabilizer angleof incidence is equal to or above a threshold stabilizer angle ofincidence, and less restrictive if the current stabilizer angle ofincidence is below the threshold stabilizer angle of incidence. Thesystem may include an elevator actuator configured to move the elevatorto a commanded elevator position that is no greater than the elevatorposition limit.

Also disclosed is a method of controlling an elevator and which mayinclude selecting a factor for increasing or decreasing a predeterminedhorizontal tail load alleviation (HTLA) authority limit for an elevatorbased on at least one aircraft parameter. The HTLA authority limit maydecrease with an increase in Mach number and/or airspeed. The method mayfurther include computing an elevator position limit as a product of theHTLA authority limit and the factor, and moving the elevator to acommanded elevator position that is no greater than the elevatorposition limit.

The features, functions and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawingsbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present disclosure will become moreapparent upon reference to the drawings wherein like numbers refer tolike parts throughout and wherein:

FIG. 1 is a block diagram of a system for controlling an elevator of anaircraft;

FIG. 2 is a plan view of an aircraft;

FIG. 3 is a sectional view taken along line 3 of FIG. 2 andschematically illustrating a wing and a horizontal tail wherein theelevator is oriented at a negative angle of incidence;

FIG. 4 is a sectional view of the wing and horizontal tail wherein theelevator is oriented at a positive angle of incidence;

FIG. 5 is a graph of an operating envelope for an aircraft andillustrating flight regimes for which movement of the elevator may belimited based upon Mach number, airspeed, dynamic pressure, and/oraltitude;

FIG. 6 is a schematic illustration of an example of a system forcomputing an elevator position limit based upon a current stabilizerangle of incidence of a stabilizer of the horizontal tail;

FIG. 7 is a schematic illustration of an example of a system forcomputing an elevator position limit based upon a current stabilizerangle of incidence and a failure of a hydraulic system of one or moreelevator actuators;

FIG. 8 is a schematic illustration of an example of a system forcontrolling an elevator of an aircraft by selecting a factor (e.g. asingle factor for both the upper and lower limit, or a different factorfor each of the upper and lower limit) for increasing or decreasing apredetermined set of horizontal tail load alleviation (HTLA) authoritylimits based upon a current stabilizer angle of incidence;

FIG. 9 is a schematic illustration of an example of a system forcontrolling an elevator wherein the factors for increasing or decreasingthe HTLA authority limits are based upon Mach number and/or airspeed;

FIG. 10 is a schematic illustration of a set of lookup tables listingfactors for different stabilizer angles of incidence based upon Machnumber and/or airspeed;

FIG. 11 is a schematic illustration of a set of lookup tables listingfactors for different levels of deployment of a speed brake handleand/or different levels of deployment of a wingmaneuvering-load-alleviation system;

FIG. 12 is a schematic illustration of an example of a system forcontrolling the elevator wherein the factors for increasing ordecreasing the HTLA authority limits are based upon the occurrence of afailure of a hydraulic system of one or more elevator actuators;

FIG. 13 is a flowchart illustrating one or more operations that may beincluded in a method of positioning an elevator based upon a currentstabilizer angle of incidence;

FIG. 14 is a flowchart illustrating one or more operations that may beincluded in a method of positioning an elevator based upon apredetermined set of HTLA authority limits.

DETAILED DESCRIPTION

Referring now to the drawings wherein the showings are for purposes ofillustrating various examples of the present disclosure, shown in FIG. 1is a block diagram of an elevator control system 400 that may beimplemented for controlling an elevator 360 of an aircraft 100. Theaircraft 100 may include a pair of wings 200 and a horizontal tail 306.Each wing 200 may include one or more leading and/or trailing edgedevices 222, 224 (e.g., flaps 226), which may be used to augment liftcharacteristics of the wing 200. In some examples, the wings 200 mayinclude leading edge devices 222 such as leading edge slats or Kruegerflaps. The wing 200 may also include one or more trailing edge devices224 such as trailing edge flaps, flaperons 230, and/or ailerons 228. Inaddition, the wings 200 may include one or more spoilers or speed brakes232 which may be mounted to a top surface of the wing 200 and for whichdeployment may be commanded by means of a speed brake handle 234 as partof flight controls (not shown) that may be manipulated by a pilot and/orby an autopilot system (not shown). The wings 200 may further include awing maneuvering-load-alleviation system 208 for reducing the wingbending moment at the wing root 210 such as by deflecting any one ormore of the speed brakes 232, leading edge devices 222, and/or trailingedge devices 224 to vary the wing camber along a spanwise direction 214and shift the center of lift of the respective wing inboard. In someexamples, the wing maneuvering-load-alleviation system 208 may beactivated or deployed during cruise flight when the aircraft 100 isperforming a maneuver such as during a turn, when the aircraft 100 issubjected to a wind gust, and/or when the load factor (e.g., g-load) onthe aircraft 100 increases during changes in pitch of the aircraft 100such as during a pitch up of the nose, as described below.

The aircraft 100 may further include a horizontal tail 306. Thehorizontal tail 306 may include a pair of horizontal stabilizers 330.Although described as horizontal stabilizers 330, the stabilizers 330may be oriented at an angle, and may not necessarily be horizontal. Forexample, the stabilizers 330 may be oriented in a slightly upwarddirection relative to horizontal (e.g., dihedral) or a slightly downwarddirection relative to horizontal (e.g., anhedral). The term horizontalstabilizer and stabilizer may be used interchangeably herein. One ormore of the stabilizers 330 may be actuated by one or more stabilizeractuators 344 such as a stabilizer jackscrew (not shown) or otherstabilizer actuation mechanism. The stabilizer actuator 344 may be anelectromechanical device although the stabilizer actuator 344 may behydraulically powered. One or more of the stabilizers 330 may include anelevator 360 which may be pivotably coupled to the stabilizer trailingedge 336. Each elevator 360 may be actuated by one or more elevatoractuators 378. In some examples, one or more of the elevator actuators378 may be configured as hydraulic actuators and may be operated as partof a hydraulic system 380 of the aircraft 100. In some examples, theelevator actuators 378 may be configured as electromechanical actuators.

As mentioned above, for pitch maneuvering of an aircraft 100, theelevator 360 may be pivotably moved upwardly or downwardly. For example,to maneuver the aircraft 100 from a nose-up attitude to a levelattitude, the elevator 360 may be pivotably deflected or positioneddownwardly (e.g., elevator trailing edge down) so that the horizontaltail 306 generates an increased amount of upload (e.g., tail load 308)to bring the nose of the aircraft 100 back down to level. To maneuverthe aircraft 100 from a nose-down attitude to a level attitude, theelevator 360 may be pivotably deflected or positioned upwardly (e.g.,elevator trailing edge up) so that the horizontal tail 306 generates anincreased amount of download (e.g., tail load 308) to bring the nose ofthe aircraft 100 back up to level. As dynamic pressure on the horizontalstabilizer 330 and elevator 360 increases with increasing airspeed, anincreased amount of upload or download is generated by the horizontaltail 306. To avoid exceeding the load-carrying capability of thehorizontal tail 306 and fuselage 104, the authority of the elevator 360may be electronically limited with increasing airspeed. Unfortunately,excessively limiting elevator authority as a function of airspeed toreduce tail loads may result in sub-optimal maneuvering capability.

In FIG. 1, the aircraft 100 may include an elevator control system 400for controlling the deflection of the elevators 360 within limits thatreduce tail loads while providing sufficient authority for pitch controlof the aircraft 100. The elevator control system 400 may control thedeflection angle or position of the elevators 360 in a manner to reducetail loads 308 on the horizontal tail 306 and/or fuselage 104 of theaircraft 100 to avoid exceeding the load-carrying capability of theload-carrying structure (not shown) of the horizontal tail 306 and/orfuselage 104. In this regard, the elevator control system 400 may reducetail loads 308 while providing sufficient pitch control for maneuveringthe aircraft 100. In some examples, the elevator control system 400 mayinclude a flight control processor 402 (e.g., a flight managementcomputer). The flight control processor 402 may receive signalsregarding the position of one or more control surfaces and the status ofone or more systems of the aircraft 100. For example, the flight controlprocessor 402 may receive signals representative of the position and/ordeployment level of the control surfaces of the wings 200 such as theposition and/or deployment level of the leading edge devices 222, thespeed brakes 232, and the trailing edge devices 224.

In some examples, the flight control processor 402 may receive signalsrepresentative of the deployment level of the wingmaneuvering-load-alleviation system 208. The wingmaneuvering-load-alleviation system 208 may include the speed brakes232, the leading edge devices 222, and/or the trailing edge devices 224,any combination of which may be deflected upwardly and/or downwardly asa means to vary the wing camber to shift the wing loading in an inboarddirection and thereby reduce wing bending at the wing root 210. Theflight control processor 402 may also receive signals representative ofthe position of a speed brake handle 234 which may be mounted on aflight deck (not shown) of the aircraft 100 and which may be manipulatedby the flight crew, as indicated above. The flight control processor 402may additionally receive signals representative of the currentstabilizer angle of incidence 338 of the horizontal stabilizer 330. Forexample, the flight control processor 402 may receive a signal from oneor more stabilizer sensors (not shown) indicative of the currentstabilizer angle of incidence 338.

Referring to FIG. 1, in some examples, the flight control processor 402may be configured to control the position of the elevators 360 by meansof one or more elevator actuators 378. As described in greater detailbelow, in some examples, the flight control processor 402 may select orcompute an elevator position limit 374, 376 (e.g., FIGS. 6-9 and 12)based on the current stabilizer angle of incidence 338. In someexamples, the elevator position limits 374, 376 may be electronicposition limits computed by the flight control processor 402. Theelevator actuators 378 may move the elevators 360 to a commandedelevator position in response to an elevator command initiated by apilot (e.g., via a control column on the flight deck) or by anautopilot. In some examples, the elevator command may be received by theflight control processor 402. The flight control processor 402 maycompare the elevator command to the elevator position limit 374, 376computed by the flight control processor 402. If the magnitude of theelevator command is within the elevator position limit 374, 376, theelevator command may be sent to the elevator actuators 378 or to acomputer or other controller integrated or embedded in the elevatoractuators 378. If the magnitude of the elevator command is greater thanthe elevator position limit 374, 376, the elevator command may bereduced to the value of the elevator position limit 374, 376 prior tothe elevator command being sent to the elevator actuators 378 or to acomputer embedded in the elevator actuators 378. In other examples, theelevator command may be sent directly to the elevator actuators and ifthe elevator command exceeds the elevator position limits 374, 376, theelevator actuators may limit movement of the elevator to the magnitudeof the elevator position limits 374, 376, in a manner described below.

FIG. 2 is a plan view of an aircraft 100 that may include any of theexamples of an elevator control system 400 disclosed herein. Theaircraft 100 may include a fuselage 104 and a longitudinal axis 106 thatmay extend lengthwise along the fuselage 104. The aircraft 100 mayinclude a lateral axis 108 oriented perpendicular the longitudinal axis106. The lateral axis 108 may pass through an aircraft center of gravity(CG) 114. The aircraft 100 may pivot about the lateral axis 108 duringchanges in the pitch attitude of the aircraft 100. The aircraft 100 mayhave a CG range 116 that may define the forward and aft limits of theaircraft CG 114. The aircraft CG 114 may shift during a flight such as aresult of fuel burn, passenger or cargo movement, and/or for otherreasons that may cause the aircraft 100 to pivot about the lateral axis108 thereby causing a change in the aircraft pitch attitude. The wingaerodynamic center 204 (FIG. 2) or center of lift may also shift forwardor aft during flight as a result of a change in speed of the aircraft100, deflection of control surfaces (e.g., leading edge devices 222,trailing edge devices 224, speed brakes 232, ailerons 228, etc.) and/orfor other reasons. The shift in the aerodynamic center 204 relative tothe aircraft CG 114 may also cause a change in the aircraft 100 pitchattitude.

The aircraft 100 may include a pair of wings 200 that may be attached tothe fuselage 104 at a wing root 210. Each one of the wings 200 mayextend outwardly in a spanwise direction 214 toward a wing tip 212. Inthe example shown, the wings 200 are swept aftwardly. In some examples,the aircraft 100 may be configured such that the aerodynamic center 204(e.g., center of lift) is located aft of the aircraft CG 114 and thepitch axis or lateral axis 108. However, the aerodynamic center 204 maybe located forward of the aircraft CG 114 and the lateral axis 108. Thewing 200 may include one or more control surfaces such as leading edgedevices 222, trailing edge devices 224, and/or speed brakes 232. Theleading edge devices 222 may include leading edge slats and/or Kruegerflaps 226 or other leading edge device configurations. The trailing edgedevices 224 may be trailing edge flaps 226, flaperons 230, ailerons 228,and/or other trailing edge device configurations. Each wing 200 may alsoinclude one or more spoilers or speed brakes 232 mounted to the topsurface of the wing 200. The aircraft 100 may include one or morepropulsion units 102 which may be mounted on the wings 200 or at otherlocations of the aircraft 100.

As shown in FIG. 2, the aircraft 100 may include an empennage 300 at theaft end of the fuselage 104. The empennage 300 may include a horizontaltail 306 and a vertical tail. The horizontal tail 306 may include one ormore horizontal stabilizers 330. Each stabilizer 330 may include anelevator 360 pivotably coupled to the stabilizer 330, as indicatedabove. The vertical tail may include a vertical stabilizer 302 and arudder 304 for directional control the aircraft 100. The horizontal tail306 may be supported by the load-carrying structure of the fuselage 104.The load-carrying structure of the fuselage 104 may include the aftfuselage body structure 112 (e.g., skin, stringers, frames, etc.) andthe fuselage center section 110 over the wings 200. Although theelevator control system 400 of the present disclosure is described inthe context of a tube-and-wing aircraft 100 as shown in FIG. 2, theelevator control system 400 may be implemented in any aircraftconfiguration, without limitation.

FIG. 3 is a sectional view of a wing 200 and a horizontal tail 306. Thewing 200 may include a wing leading edge device 222 and a wing trailingedge device 224. The wing 200 may include one or more speed brakes 232shown in a retracted position. In some examples, the wing aerodynamiccenter 204 (e.g., center of lift) may be located aft of the aircraft CG114. The wing 200 may generate positive wing lift 202 which may producea wing pitching moment 206 about the aircraft CG 114. In examples inwhich the aircraft CG 114 is located forward of the aerodynamic center204, the positive wing lift 202 of the wings 200 may generate anose-down wing pitching moment 206. In further examples, the aircraft CG114 may be located aft of the aerodynamic center 204 in which case thepositive wing lift 202 of the wings 200 may generate a nose-up wingpitching moment 206. The wing 200 may have a wing chord 216 extendingfrom the wing leading edge 218 to the wing trailing edge 220, and whichmay be used to describe the orientation of the wing 200 relative to thelongitudinal axis 106. In the example shown, the wing chord 216 isoriented parallel to the longitudinal axis 106 (e.g., the angle ofincidence is zero). However, the wing chord 216 may be oriented at anangle of incidence (not shown) relative to the longitudinal axis 106.

In FIG. 3, the horizontal tail 306 includes a horizontal stabilizer 330.The horizontal stabilizer 330 has a stabilizer leading edge 334 and astabilizer trailing edge 336 defining a stabilizer chord 332. In FIG. 3,the stabilizer chord 332 is oriented at a negative angle of incidencerelative to the longitudinal axis 106. As indicated above, thestabilizer 330 may include one or more stabilizer actuators 344 that maybe operated to adjust the stabilizer angle of incidence 338 tocounteract the pitching moment 206 generated by the wings 200. Thestabilizer 330 includes an elevator 360. The elevator 360 may bepivotably coupled to the stabilizer 330. The elevator 360 is shown insolid lines in the neutral position 368. In the neutral position 368,the elevator chord 362 is aligned with the stabilizer chord 332. One ormore elevator actuators 378 may be commanded to pivotably move theelevator 360 between an upper elevator position limit 374 and a lowerelevator position limit 376. The upper and lower elevator positionlimits 374, 376 may be determined by a flight control processor 402, asdescribed in greater detail below. When the horizontal stabilizer 330 isoriented at a negative stabilizer angle of incidence 338, the horizontaltail 306 may generate negative lift or a download 312 which may bedescribed as acting at a horizontal tail aerodynamic center 314. Thehorizontal stabilizer 330 and elevator 360 may be adjusted such that thedownload 312 provides a nose-up horizontal tail moment 316 to counteracta nose-down wing pitching moment 206 generated by the positive wing lift202 of the wings 200 such as when the wing aerodynamic center 204 islocated aft of the aircraft CG 114.

FIG. 4 is a sectional view of a wing 200 and a horizontal tail 306. Thewing 200 is shown with the speed brake 232 deployed. The stabilizerchord 332 is oriented at a positive angle of incidence relative to thelongitudinal axis 106. The elevator 360 is shown in solid lines in theneutral position 368. With the horizontal stabilizer 330 oriented at apositive stabilizer angle of incidence 338, the horizontal tail 306 maygenerate positive wing lift 202 or an upload 310 acting at thehorizontal tail aerodynamic center 314. The horizontal stabilizer 330and/or the elevator 360 may be adjusted such that the upload 310provides a nose-down horizontal tail moment 316 to counteract a nose-upwing pitching moment 206 generated by the positive wing lift 202 of thewings 200 such as when the wing aerodynamic center 204 is located aft ofthe aircraft CG 114 and/or due to a nose-up pitching moment generated bythe speed brakes 232 when deployed.

FIG. 5 shows a graph of an example operating envelope of an aircraft 100and illustrating flight regimes or critical loads regions for whichmovement of the elevator 360 may be limited based upon Mach number 414,airspeed 410, dynamic pressure 416, and/or altitude 412, as describedbelow. In the example shown, the critical loads regions include a firstcritical loads region 382 and a second critical loads region 384. Thecritical loads regions 382, 384 may be determined based upon strengthanalyses and/or testing to identify the load-carrying capability of thehorizontal tail 306 and fuselage 104 in response to tail loads 308 atthe noted operating parameters (e.g., Mach number 414, airspeed 410,dynamic pressure 416, and altitude 412) and wherein such tail loads 308on the aircraft 100 may approach the load-carrying capability of thehorizontal tail 306 and/or fuselage 104. In the example graph shown, thefirst and second critical loads regions 382, 384 may be based onairspeed 410 and Mach number 414. However, any one or more of theparameters of Mach number 414, airspeed 410, dynamic pressure 416,and/or altitude 412, taken alone or in any combination with one another,may be used to define a critical loads region. For example, a criticalloads region may be defined by (e.g., bounded by) dynamic pressure(e.g., qbar) 416 and Mach number 414, or a critical loads region may bedefined by altitude 412 and airspeed 410, or any other combination ofMach number 414, airspeed 410, dynamic pressure 416, and altitude 412.In the example shown, the first and second critical loads regions 382,384 have a respective reduction factor of 0.8 and 0.7 which may be usedfor computing the upper and lower elevator position limits 374, 376, asdescribed below. As may be appreciated, the operating envelope of anaircraft 100 may include any number of critical loads regions. Thereduction factor of each critical loads region may have any value,without limitation.

FIG. 6 shows an example of an elevator control system 400 wherein theelevator 360 position is adjusted as a function of the current angle ofincidence 338 of the horizontal stabilizer 330. The elevator controlsystem 400 may limit the elevator 360 position (e.g., the range ofmotion of the elevator) as a means to avoid excessive tail loads 308 inthe horizontal tail 306 and/or in the fuselage 104 (FIG. 1) whileproviding sufficient nose-up and nose-down maneuverability of theaircraft 100 (FIG. 1) to maintain pitch control of the aircraft 100. Inthe present disclosure, the tail loads 308 may be described as the loadstransmitted through the load-carrying structure of the horizontal tail306 including the stabilizer 330 and the elevator 360. In addition, thetail loads 308 may include loads transmitted through the load-carryingstructure of the aft fuselage body structure 112 (FIG. 1) including thefuselage skin, longitudinal stringers, and circumferential frames, andmay further include the loads in the fuselage center section 110 such asin the over-wing body structure of the fuselage 104. In addition, thetail loads 308 may include structural loads imposed on or carried by thestabilizer actuators 344 and the elevator actuators 378, and may furtherinclude loads on hinges, pivots, and any other structure or equipmentthat may transmit tail loads 308.

In FIG. 6, the flight control processor 402 may receive a stabilizersignal representative of the current stabilizer angle of incidence 338.In some examples, the current stabilizer angle of incidence 338 may bedescribed as a proxy for the position of the speed brakes 232 in thesense that the current stabilizer angle of incidence 338 may beindicative of whether the speed brakes 232 are retracted or deployed. Insome examples, deployment of the speed brakes 232 may represent anaircraft 100 configuration wherein the horizontal tail 306 may besusceptible to relatively high tail loads 308 in comparison to tailloads 308 when the speed brakes 232 are retracted. The flight controlprocessor 402 may receive the stabilizer signal from one or morestabilizer sensors, and compute an elevator position limit 374, 376based on the current stabilizer angle of incidence 338.

The elevator position limit 374, 376 may be described as the position(e.g., the deflection angle) of the elevator 360 relative to the neutralposition 368 (FIGS. 3-4) of the elevator 360. The flight controlprocessor 402 may compute an upper elevator position limit 374 and alower elevator position limit 376, and which may be transmitted to oneor more elevator actuators 378 such as via the flight control processor402. An elevator actuator 378 may pivotably move the elevator 360 to acommanded elevator position that is no greater than the upper and lowerelevator position limits 374, 376. The commanded elevator position maybe commanded by a pilot and/or an autopilot using an elevator controlinput 406 device in communication with the elevator actuators 378. Theelevator control input 406 device may be a control column (not shown)located on a flight deck (not shown) of an aircraft 100. In someexamples, the flight control processor 402 may compare a pilot-initiatedor autopilot-initiated elevator command to the upper and lower elevatorposition limits 374, 376, and if the elevator command exceeds the upperand lower elevator position limits 374, 376, the flight controlprocessor 402 may reduce the magnitude of the elevator command prior tosending the elevator command to the elevator actuators 378. In furtherexamples, the elevator command may be sent directly to the elevatoractuators 378 using an elevator control input 406 device. Prior toactuation of the elevators 360, the elevator actuators 378 may reducethe magnitude of the elevator command to a magnitude that does notexceed the upper and lower elevator position limits 374, 376, asdescribed below.

As indicated above, the horizontal stabilizer 330 may be positioned at acurrent stabilizer angle of incidence 338 such that the horizontal tailmoment 316 counteracts the wing pitching moment 206. In this manner, theaircraft 100 may be trimmed to maintain a substantially constant pitchangle. The current stabilizer angle of incidence 338 may be based on oneor more aircraft parameters. For example, the current stabilizer angleof incidence 338 may be based on airspeed, Mach number, dynamicpressure, current aircraft gross weight, current aircraft CG location,current aircraft pitch rate, speed brake position (e.g., retracted ordeployed, and angle of deployment), thrust setting of the propulsionunits 102, activation status of a wing maneuvering-load-alleviationsystem 208 (e.g., inactive or active, and level of deployment), and anyone of a variety of other aircraft parameters upon which the currentstabilizer angle of incidence 338 may be based. The current stabilizerangle of incidence 338 may be commanded by a pilot or an autopilot usinga stabilizer control device (not shown) in communication with the flightcontrol processor 402 and/or in communication with one or morestabilizer actuators 344 (FIG. 1).

As shown in FIG. 6, the flight control processor 402 may receive athreshold stabilizer angle of incidence 342, which may be stored in adate storage device of aircraft 100 (e.g., storage device 403 of theflight control computer 401 of aircraft 100). One or more thresholdstabilizer angles of incidence 342 may be stored, each associated with agiven flight condition, aircraft type, aircraft configuration and/or oneor more flight parameters. The flight control processor 402 may comparethe threshold stabilizer angle of incidence 342 with the currentstabilizer angle of incidence 338, which may be above or below thethreshold stabilizer angle of incidence 342. The threshold stabilizerangle of incidence 342 may be described as an angle of incidence thatrepresents whether the speed brakes 232 are retracted (see e.g., FIG. 3)or deployed (see e.g., FIG. 4).

In this regard, the threshold stabilizer angle of incidence 342 may bedescribed as the value below which the speed brakes 232 are presumed tobe retracted for purposes of computing elevator position limits 374,376. For example, the threshold stabilizer angle of incidence 342 may bepositive 1 degree relative to the longitudinal axis 106 (FIG. 1) of theaircraft 100. In such examples, a current stabilizer angle of incidence338 which is less than positive 1 degree may represent that the speedbrakes 232 are retracted. A current stabilizer angle of incidence 338 ofpositive 1 degree or higher may represent that the speed brakes 232 aredeployed. In further examples, the threshold stabilizer angle ofincidence 342 may be different than positive 1 degree. For example, thethreshold stabilizer angle of incidence 342 for one type of aircraft maybe positive 2 degrees relative to the longitudinal axis 106 of theaircraft 100, and the threshold stabilizer angle of incidence 342 foranother type of aircraft may be 0 degrees relative to the longitudinalaxis 106 of the aircraft 100. Deployment of the speed brakes 232 maygenerate a nose-up pitching moment which may reduce the nose-down wingpitching moment 206 generated by the wing lift 202. In this regard, whenthe speed brakes 232 are deployed, the stabilizer 330 may be positionedat a stabilizer angle of incidence 338 that results in the horizontaltail 306 generating a smaller amount of download 312, or generating anupload 310, to counteract a reduction in nose-down wing pitching moment206 when the speed brakes 232 are deployed.

The flight control processor 402 may compare the current stabilizerangle of incidence 338 with the threshold stabilizer angle of incidence342 and select an elevator position limit 374, 376 that is morerestrictive if the current stabilizer angle of incidence 338 is equal toor greater than the threshold stabilizer angle of incidence 342, andless restrictive if the current stabilizer angle of incidence 338 isbelow the threshold stabilizer angle of incidence 342. The flightcontrol processor 402 may compute a plurality of elevator positionlimits 374, 376 which may be stored in lookup tables (e.g., lookuptables 418, as illustrated in the example in FIG. 6). The flight controlprocessor 402 may compute upper and lower elevator position limits 374,376 for any one parameter or any given combination of parameters (e.g.,Mach number, airspeed, etc.) and may store the upper and lower elevatorposition limits 374, 376 in respective lookup tables 418 (e.g., anelevator-up lookup table and an elevator-down lookup table). In furtherexamples, the plurality of elevator position limits 374, 376 may becomputed in advance (e.g., prior to flight) and may be preprogrammedinto a flight control computer 401 (e.g., stored in lookup tables instorage device 403) for access by the flight control processor 402during flight. The values of the upper and lower elevator positionlimits 374, 376 may be based upon the current stabilizer angle ofincidence 338 in that less restrictive values may be computed for theelevator position limits 374, 376 if the current stabilizer angle ofincidence 338 is less than the threshold stabilizer angle of incidence342, and more restrictive values may be computed for the elevatorposition limits 374, 376 if the current stabilizer angle of incidence338 is greater than or equal to the threshold stabilizer angle ofincidence 342. Pairs of lookup tables may be generated for the pluralityof sets of upper elevator position limits and lower elevator positionlimits. For example, a first pair of lookup tables may contain upperelevator position limits and a second pair of lookup tables may containlower elevator position limits. The first and second pair of lookuptables in FIG. 6 may each include a first table of elevator positionlimits (e.g., elev lim_up-(1), elev lim_down-(1)) for situations inwhich the current stabilizer angle of incidence is less than thethreshold stabilizer angle of incidence, and a second table of elevatorposition limits (e.g., elev lim_up_(restricted)-(1), elevlim_down_(restricted)-(1)) for situations in which the currentstabilizer angle of incidence is greater than or equal to the thresholdstabilizer angle of incidence.

In FIG. 6, upper elevator position limits 374 and lower elevationposition limits 376 may be selected based upon the value of the currentstabilizer angle of incidence 338 relative to the threshold stabilizerangle of incidence 342 for a given Mach number and airspeed. That is, aplurality of upper and lower elevator position limits 374, 376 may becomputed, each for a given combination of airspeed and Mach number. Thespecific examples of values for Mach number and airspeed in FIG. 6, aswell as in FIGS. 7-12, do not represent an exclusive list of possiblevalues but are provided for illustration purposes only. In otherexamples, the Mach number and airspeed values may be different. Infurther examples, as described above, the flight control processor 402may compute the upper and lower elevator position limit 374, 376 basedupon any combination of parameters including Mach number, airspeed,altitude, dynamic pressure, or any combination thereof, and is notlimited to computing the upper and lower elevator position limit 374,376 based upon Mach number and airspeed. For example, the upper andlower elevator position limit 374, 376 may be computed based solely onMach number, or airspeed, or altitude. In another example, the upper andlower elevator position limit 374, 376 may be computed based solely ondynamic pressure.

In any of the examples disclosed herein, airspeed may be described asthe equivalent airspeed of the aircraft or the calibrated airspeed ofthe aircraft. Equivalent airspeed may be described as the speed of anaircraft at sea level that would produce the same magnitude of dynamicpressure as the dynamic pressure at the true airspeed and altitude atwhich the aircraft is flying. Calibrated airspeed may be described asthe indicated airspeed (e.g., as indicated by an airpseed indicator onan instrument panel) corrected for instrument errors, and corrected forposition errors and installation errors at the airspeed sensor (e.g., ata pitot tube mounted on the aircraft exterior).

Referring still to FIG. 6, the stabilizer signal may include a magnitudeof the current stabilizer angle of incidence 338. The flight controlprocessor 402 may receive the stabilizer position signal from one ormore stabilizer sensors (not shown) and may compare the magnitude of thecurrent stabilizer angle of incidence 338 with the threshold stabilizerangle of incidence 342. The flight control processor 402 may selectand/or compute in real-time elevator position limits 374, 376 that aremore restrictive for relatively high values of the current stabilizerangle of incidence 338, and less restrictive for relatively low valuesof the current stabilizer angle of incidence 338. For the example, for agiven Mach number and/or airspeed, for a current stabilizer angle ofincidence 338 of positive 5 degrees, the flight control processor 402may select an upper elevator position limit 374 of +10 degrees and alower elevator position limit 376 of −10 degrees. However, for a currentstabilizer angle of incidence 338 of positive 1 degree, the flightcontrol processor 402 may select an upper elevator position limit 374 of+18 degrees and a lower elevator position limit 376 of −18 degrees.

In some examples, the elevator position limits 374, 376 may beproportional to the magnitude of the current stabilizer angle ofincidence 338. In some cases, the elevator position limits 374, 376 maybe linearly proportional to the magnitude of the current stabilizerangle of incidence 338. In yet further examples, the elevator positionlimits 374, 376 may generally decrease (e.g., may be more restrictive)with increasing airspeed for a given Mach number, or the upper and/orlower elevator position limits 374, 376 may increase (e.g., may be lessrestrictive) for an increase in airspeed for a given Mach number. Theupper and/or lower elevator position limits 374, 376 may likewiseincrease (e.g., may be less restrictive) for an increase in dynamicpressure, altitude, or any combination of Mach number, airspeed, dynamicpressure, and altitude.

In some examples, for a given Mach number and airspeed, the absolutevalue (also referred to herein as magnitude) of the elevator positionlimits may be the same in both the up and down direction. However thatneed not be the case as will be described further below. As describedherein, the absolute value of the elevator position limits 374, 376 forthe case where the current stabilizer angle of incidence 338 is lessthan the threshold stabilizer angle of incidence 342, are higher thanthe absolute value of the elevator position limits 374, 376 for the casewhere the current stabilizer angle of incidence 338 is equal to orgreater than the threshold stabilizer angle of incidence 342. Theelevator position limits 374, 376 may be the same or they may bedifferent for different Mach numbers for a given airspeed. For example,the flight control processor 402 may compute more restrictive elevatorposition limits 374, 376 as Mach number increases and/or lessrestrictive elevator position limits 374, 376 as Mach number increases.The flight control processor 402 may also compute elevator positionlimits 374, 376 solely as a function of Mach number, or airspeed, ordynamic pressure, or altitude. The flight control processor 402 may alsocompute elevator position limits 374, 376 as a function of anycombination of the parameters of Mach number, airspeed, dynamicpressure, and altitude.

In further examples, the flight control processor 402 may also beconfigured to compute and/or select elevator position limits 374, 376that are more restrictive in the direction of the current stabilizerangle of incidence 338, which may serve as means to reduce tail loads308 relative to the tail loads 308 generated in an arrangement where theelevator position limits 374, 376 have the same value in the positivedirection as in the negative direction. Due to the tail lift (e.g.,positive or negative) generated by the horizontal tail 306 being greaterin the direction of orientation of the stabilizer 330, the elevatorposition limits 374, 376 may be more restrictive in the direction of thecurrent stabilizer angle of incidence 338. In this manner, tail loads308 may be reduced while the elevator position limits 374, 376 provideenough pitch maneuverability to allow the aircraft 100 to recover from anose-up or nose-down pitch attitude. In some examples, the flightcontrol processor 402 may compute and/or select elevator position limits374, 376 that are more restrictive for a higher magnitude (e.g.,absolute value) of the current stabilizer angle of incidence, andcompute elevator position limits 374, 376 that are less restrictive fora lower magnitude (e.g., absolute value) of the current stabilizer angleof incidence.

In some examples, the magnitude of the upper elevator position limit 374may be different from the magnitude of the lower elevator position limit376. For example, if the current stabilizer angle of incidence 338 isabove the threshold stabilizer angle of incidence 342, the flightcontrol processor 402 may compute and/or select elevator position limits374, 376 that are more restrictive in the positive direction than in thenegative direction. In an example, if the current stabilizer angle ofincidence 338 is higher than the threshold stabilizer angle of incidence342, the flight control processor 402 may compute an upper elevatorposition limit 374 of 8 degrees for an aircraft 100 moving at Mach 0.6and airspeed 600 miles per hour (mph), and a lower elevation positionlimit of 10 degrees for the same Mach and airspeed. If the currentstabilizer angle of incidence 338 is lower than the threshold stabilizerangle of incidence 342, the flight control processor 402 may compute anelevator position limit 374, 376 that is more restrictive in thenegative direction than in the positive direction. For example, if thecurrent stabilizer angle of incidence 338 is lower than the thresholdstabilizer angle of incidence 342, the flight control processor 402 maycompute and/or select an upper elevator position limit 374 of 10 degreesfor an aircraft 100 moving at Mach 0.6 and airspeed 600, and a lowerelevation position limit 376 of 8 degrees for the same Mach andairspeed.

In some examples, an elevator command initiated by a pilot or autopilotmay be sent directly to the elevator actuators 378, bypassing the flightcontrol processor 402. The elevator command may be reduced based on thestabilizer angle of incidence 342, and based on Mach number, airspeed,dynamic pressure, and/or altitude. The elevator command may be sentdirectly to one or more of the elevator actuators 378 to reduce thehydraulic pressure capability or blow-down limit inside the hydraulicelevator actuators 378. In some examples, the reduction factor may becomputed by an elevator computer (not shown) in response to inputsregarding stabilizer angle of incidence, Mach number, airspeed, dynamicpressure, and/or altitude. The reduction in the hydraulic pressurecapability of the elevator actuators 378 may be analogous to theabove-described electronic position limit of the elevators, and may beimplemented in the event of the outage of a main computer of theaircraft 100, or other condition of the aircraft 100.

FIG. 7 shows an elevator control system 400 according to furtherexamples of the present disclosure. The elevator control system 400 maybe operable to compute elevator position limits 374, 376 based upon thecurrent stabilizer angle of incidence 338 in a manner similar to theelevator control system 400 shown in FIG. 6 and described above. Theelevator control system 400 shown in FIG. 7 is further configured toaccommodate a failure of one or more hydraulic systems 380 (e.g.,hydraulic systems described previous with respect to FIGS. 1-3). Forexample, the aircraft 100 may include left and right elevators 360 whichmay each include a dedicated, independent hydraulic elevator actuator378 that may be in fluid communication with a central hydraulic system(not shown). Each one of the hydraulic actuators may be configured suchthat if hydraulic pressure is lost such as due to loss of a propulsionunit 102 (FIG. 1), one or more of the hydraulic actuators may remainoperative to provide the ability to move at least one of the elevators360 (e.g., the left elevator or the right elevator) so that pitchcontrol of the aircraft 100 can be maintained. In such a scenario, thedeflection capability of one or both of the elevators 360 may beincreased at the risk of temporarily exceeding the design-allowableload-carrying capability of the horizontal tail 306 and/or fuselage 104.In some examples, it may be desirable to increase the deflectioncapability of the elevator on one side of the vertical tail (e.g., theleft elevator) to increase maneuvering capability because the elevatoron the opposite side of the vertical tail (e.g., the right elevator) isnot working properly or is non-functional. In still other examples, itmay be desirable to decrease the deflection capability of one elevator(e.g., the left elevator) if the opposite elevator (e.g., the rightelevator) is non-functional in order to avoid a structural failure ofthe tail due to a relatively high rolling moment due to asymmetricalelevator input. As illustrated in FIG. 7, the elevator control system400 may be configured to select upper and/or lower elevator positionlimits 374, 376 which take into account a failure of one or morehydraulic systems. The flight control processor may access lookup tables420 which include elevator position limits 374, 376 associated with ahydraulic system failure scenario. The elevator position limits 374, 376associated with a hydraulic system failure scenario (e.g., elevlim_up-(2), elev lim_down-(2), elev lim_up_(reduced)-(2), elev lim₁₃down_(reduced)-(2)) may be less restrictive than elevator positionlimits 374, 376 associated with the non-failure scenario shown in FIG.6. The flight control processor 402 may be further configured to selectan elevator position limit 374, 376 from lookup table 420 in response toreceiving a signal or indication of a hydraulic system failure.

FIG. 8 is a schematic illustration of a further embodiment of anelevator control system 400 for controlling an elevator 360 of ahorizontal tail 306 of an aircraft 100. The system 400 of FIG. 8 may beoperated based upon a predetermined set of horizontal tail loadalleviation (HTLA) authority limits which may be scheduled according toMach and/or airspeed. The HTLA authority limits may be computed usingknown load alleviation processes for limiting movement of controlsurfaces based on a loading of the control surface, for example asdescribed in U.S. Pat. No. 8,342,445, assigned to applicant andincorporated herein by reference in its entirety for any purpose. TheHTLA authority limits may be stored in a set of lookup tables 422. TheHTLA authority limits 370, 372 may be described as electronic limitsthat restrict the deflection capability of the elevators 360 from theirmaximum positions which may be determined by mechanical limits or stops(not shown) for the elevators 360. The HTLA authority limits 370, 372may increasingly restrict elevator movement (e.g., deflection angle)with increasing Mach and/or airspeed as the effectiveness of theelevator 360 increases due to increasing dynamic pressure. In thismanner, the elevator control system 400 may prevent movement of theelevator 360 to excessively high deflection angles that may result intail loads 308 approaching the structural load-carrying capability ofthe tail, the tail supporting structure, the fuselage, and otherstructures and equipment including, but not limited to, the stabilizeractuators 344 and elevator actuators 378 and associated hardware.

In FIG. 8, the flight control processor 402 may be preprogrammed withthe predetermined set of upper and lower HTLA authority limits 370, 372as listed in the lookup tables 422 (e.g., elev lim_up-(3), elevlim_down-(3)). In some examples, the flight control processor 402 maycompute the HTLA authority limits 370, 372 in real time (e.g., duringflight) and store the computed HTLA authority limits in lookup tables422. The flight control processor 402 may select a factor for increasingor decreasing the predetermined set of HTLA authority limits 370, 372for different elevator 360 positions based upon one or more aircraftparameters. The flight control processor 402 may compute upper and lowerelevator position limits 374, 376 as a product of the factor and theupper and lower HTLA authority limits 370, 372. The upper and lowerelevator position limits 374, 376 may be transmitted to one or moreelevator actuators 378. A commanded elevator position may be commandedby a pilot or an autopilot using an elevator control input 406 device incommunication with the elevator actuators 378, as described above. Theone or more elevator actuators 378 may move the elevator 360 to acommanded elevator position that is no greater than the upper and lowerelevator position limits 374, 376 such that excessive tail loads 308 areavoided while retaining pitch maneuverability for the aircraft 100.

In the example of FIG. 8, the aircraft parameter 408 upon which thefactor is based is the current stabilizer angle of incidence 338 of thestabilizer 330. As indicated above, the current stabilizer angle ofincidence 338 may be described as a proxy for the position of the speedbrakes 232. The flight control processor 402 may receive a signalrepresentative of the current stabilizer angle of incidence 338, and maycompute a factor for the current stabilizer angle of incidence 338. FIG.8 shows a lookup table 424 containing a listing of factors, each ofwhich may correspond to, or which may be computed for, a givenstabilizer angle of incidence 338. In some examples, the magnitude ofthe factor may increase with an increase in the current stabilizer angleof incidence 338 as a means to limit the tail loads 308 as the currentstabilizer angle of incidence 338 moves in a positive direction. In someembodiments, a single set of factors may be provided for calculatingboth the upper and lower elevator position limit 374, 376. In otherembodiments, one set of factors may be provided for determining theupper elevator position limit 374, and a different set of factors may beprovided for determining the lower elevator position limit 376.

Although the factors listed in the lookup tables 424 of FIG. 8 are basedupon the current stabilizer angle of incidence 338, in a furtherembodiment not shown, the elevator control system 400 may be configuredsuch that the factors are based upon the deployment setting of one ormore other types of control surfaces, and are not limited to factorsbased upon the current stabilizer angle of incidence 338. For example,the elevator control system 400 may include a lookup table (not shown)of factors based upon the position of one or more wing leading edgedevices 222, one or more wing trailing edge devices 224, or acombination of any of a variety of leading and trailing edge devices222, 224 (FIG. 1-2) or other control surfaces that may have an effect onthe wing pitching moment 206 and may therefore affect the magnitude ofthe tail loads. For example, the factors may be based at least in partupon the position of the speed brakes 232 (see FIGS. 1-2) mounted on thewings 200. As described above, deployment of the speed brakes 232 on aswept wing aircraft may generate a nose-up wing pitching moment 206 (seeFIG. 4). The elevator control system 400 may be configured to compute orapply one or more factors to the upper and lower HTLA authority limits370, 372 to determine the upper and lower elevator position limits 374,376 based on the deployment setting of the speed brakes 232 alone, or incombination with the current stabilizer angle of incidence 338, or anyone or various combinations deployment setting for other wing controlsurfaces such as speed brakes 232, leading edge devices 222, andtrailing edge devices 224, or any other aircraft surface(s) or device(s)that may affect the wing pitching moment 206.

FIG. 9 shows a system 400 for controlling an elevator 360 wherein thefactors for increasing or decreasing the HTLA authority limits 370, 372listed in the lookup tables 422 are based upon Mach and/or airspeed if apredicted tail load 308 is determined to exceed a load-carryingcapability of the horizontal tail 306. In this regard, the flightcontrol processor 402 may calculate the predicted tail load 308 that maybe imposed on the horizontal tail 306 with the elevator 360 at a givenHTLA authority limit 370, 372. The predicted tail load 308 may be basedon one or more of a variety of aircraft parameters including, but notlimited to, current aircraft gross weight, current aircraft CG location,current aircraft pitch rate, airspeed, wing angle of attack, controlsurface deflection (e.g., leading and/or trailing edge devices), speedbrake position and level of deployment, landing gear position, and otheraircraft parameters. The flight control processor 402 may determinewhether the predicted tail load 308 exceeds the load-carrying capabilityof the horizontal tail 306 for the given set of aircraft parameters. Theflight control processor 402 may compute or select a factor from alookup table 426 for applying to the HTLA authority limits 370, 372based on the Mach number and/or airspeed of the aircraft 100 if it isdetermined that the predicted tail load 308 may exceed the load-carryingcapability of the horizontal tail 306 and/or fuselage 104.

FIG. 10 shows a set of lookup tables 428 listing different factors fordifferent stabilizer settings (angles of incidence 338) based upon Machnumber and/or airspeed. The lookup tables 428 in FIG. 10 may bedescribed as a combination of the factor lookup tables 424, 426 shownrespectively in FIGS. 8 and 9 and described above. FIG. 10 represents anembodiment of the elevator control system 400 enabling scheduling of theelevator position limit 374, 376 as a function of current stabilizerangle of incidence 338, Mach number, and/or airspeed. The factors maygenerally increase (e.g., become more restrictive) with an increase inairspeed, although for some flight regimes, the factors may notnecessarily increase with increasing Mach number, as shown in the lookuptables 428.

FIG. 11 shows a set of lookup tables 430 listing different factors fordifferent levels of deployment of a speed brake handle 234 and/ordifferent levels of deployment of a wing maneuvering-load-alleviationsystem 208. As indicated earlier, the speed brakes 232 may be actuatedin response to manipulation of a speed brake handle 234 (FIG. 1) by apilot. For example, the pilot may move the speed brake handle 234 from aretracted detent (not shown) to a flight detent (not shown) which mayrepresent a 50 percent deployment level of the speed brakes 232 andwhich may generate a nose-up pitching moment that may reduce or add tothe wing pitching moment 206, depending upon the location of the wingaerodynamic center 204 (e.g., center of lift) relative to the aircraftCG 114. If the predicted tail load 308 is determined to exceed theload-carrying capability of the horizontal tail 306, the flight controlprocessor 402 may compute the upper and lower elevator position limits374, 376 as a product of the reduction factor and the upper and lowerHTLA authority limits 370, 372. As shown in the lookup tables 430 ofFIG. 11, the reduction factor may be computed by the flight controlprocessor 402 based upon Mach and airspeed. The upper and lower elevatorposition limits 374, 376 may reduce the tail loads 308 while preservingpitch maneuverability of the aircraft 100.

In some examples, the wings 200 (FIG. 2) may further include a wingmaneuvering-load-alleviation system 208 which may be actuated to adesired deployment level during cruise flight to reduce wing bendingmoment by deflecting one or more of the speed brakes 232, leading edgedevices 222, and/or trailing edge devices 224 to vary the wing camberalong a spanwise direction 214 (FIG. 2). The flight control processor402 may be configured to compute or look up factors in a set of lookuptables 430 which may correspond to the level of deployment of the wingmaneuvering-load-alleviation system 208 and based upon the Mach and/orairspeed of the aircraft 100. The flight control processor 402 maydetermine whether the predicted tail load 308 would exceed theload-carrying capability of the horizontal tail 306 and, if so, theflight control processor 402 may compute the upper and lower elevatorposition limits 374, 376 using the appropriate reduction factor for agiven level of deployment of the wing maneuvering-load-alleviationsystem 208.

FIG. 12 shows a further embodiment of an elevator control system 400using factors for increasing the HTLA authority limits 370, 372 listedin the lookup tables 422. The factors may be based upon a failure of oneor more hydraulic system 380 (FIG. 1) of one or more of the elevatoractuators 378. Although the lookup table 432 shows a single factor to beused upon an indication of a hydraulic system failure, different factorsmay be used for different Mach numbers and airspeeds of the aircraft.Upon the detection of a hydraulic system failure of one or more of theelevator actuators 378, the flight control processor 402 may compute afactor or select a factor from a lookup table 432 to apply to the HTLAauthority limit 370, 372. The factors may be less restrictive (e.g.,allow more deflection capability) to provide sufficient pitch control tothe aircraft 100 during a failure of one or more hydraulic systems. Insuch a scenario, the deflection capability of one or both of theelevators 360 may be increased at the risk of temporarily exceeding thedesign-allowable load-carrying capability of the horizontal tail 306and/or fuselage 104 so that pitch control of the aircraft may bemaintained.

FIG. 13 is a flowchart illustrating one or more operations that may beincluded in a method 500 of positioning an elevator 360 based upon acurrent stabilizer angle of incidence 338. With reference to FIG. 6,Step 502 of the method may include identifying the current stabilizerangle of incidence 338. As indicated above, the flight control processor402 may receive a signal representative of the current stabilizer angleof incidence 338. The current stabilizer angle of incidence 338 may berepresentative of whether the speed brakes 232 are retracted ordeployed, and may be determined based upon one or more of a variety ofaircraft parameters 408 including, but not limited to, airspeed, Machnumber, dynamic pressure, current aircraft gross weight, currentaircraft CG location, current aircraft pitch rate, speed brake position,activation status of a wing MLA system, and other aircraft parameters.

Step 504 of the method may include comparing the current stabilizerangle of incidence 338 with a threshold stabilizer angle of incidence342 to determine whether the current stabilizer angle of incidence 338is above or below the threshold stabilizer angle of incidence 342. Asindicated above, the threshold stabilizer angle of incidence 342 maycorrespond to a value below which the speed brakes 232 of the aircraft100 may be retracted.

For example, a current stabilizer angle of incidence 338 which is lessthan positive 1 degree may represent that the speed brakes 232 areretracted, and a current stabilizer angle of incidence 338 of positive 1degree or higher may represent that the speed brakes 232 are deployed.However, the threshold stabilizer angle of incidence 342 may bedifferent than positive 1 degree.

Step 506 of the method 500 may include selecting an elevator positionlimit 374, 376 based on the current stabilizer angle of incidence 338.In some examples, the method may include generating a plurality oflookup tables 418 including a first pair of lookup tables containingvalues for an upper elevator position limit 374 and a second pair oflookup tables containing values for a lower elevator position limit 376.As indicated above, the elevator position limits may be computed as afunction of Mach number, airspeed, dynamic pressure, and/or altitude,and may be based on a threshold stabilizer angle of incidence 342. Theelevator position limits may be selected from the lookup tables 418. Insome examples, a flight control processor 402 (FIG. 1) may compute theelevator position limits 374, 376 and store the elevator position limits374, 376 in lookup tables 418 of a storage device 403 (FIG. 1). In otherexamples, elevator position limits 374, 376 may be computed in advance(e.g., prior to a flight) and may be preprogrammed into a flight controlcomputer 401 and stored in the storage device 403 for access by theflight control processor 402 during flight.

In Step 506 the method may include selecting an elevator position limit374, 376 that is more restrictive if the current stabilizer angle ofincidence 338 is equivalent to or above a threshold stabilizer angle ofincidence 342, and selecting an elevator position limit 374, 376 that isless restrictive if the current stabilizer angle of incidence 338 isbelow the threshold stabilizer angle of incidence 342. For example, ifthe current stabilizer angle of incidence 338 is greater than or equalto the threshold stabilizer angle of incidence 342, a value for an upperelevator position limit 374 may be selected from a first table of thefirst pair of lookup tables, and if the current stabilizer angle ofincidence 338 is below the threshold stabilizer angle of incidence 342,a value for the upper elevator position limit 374 may be selected from asecond table of the first pair of lookup tables.

In some examples, an upper elevator position limit 374 may be associatedwith a given airspeed and Mach number of an aircraft 100. A lowerelevator position limit 376 may be selected from a second pair of lookuptables. The lower elevator position limit 376 may be associated with thesame airspeed and Mach number as the upper elevator position limit 374.However, the lower elevator position limit 376 may have a differentmagnitude than the magnitude of the upper elevator position limit 374.For example, an aircraft 100 moving at Mach 0.6 with an airspeed of 600miles per hour may have an upper elevator position limit 374 of 8degrees and a lower elevation position limit of 10 degrees for the sameMach and airspeed.

In some examples, the selection of an elevator position limit 374, 376may include selecting a lower elevator position limit 376 from a secondpair of lookup tables. The lower elevator position limit 376 may beassociated with a given airspeed and Mach number. The magnitude of thelower elevator position limit 376 may be greater than the magnitude ofthe upper elevator position limit 374 if the current stabilizer angle ofincidence 338 is negative, and the magnitude of the lower elevatorposition may be less than the magnitude of the upper elevator positionlimit 374 if the current stabilizer angle of incidence 338 is positive.For example, for an aircraft 100 moving at Mach 0.6 and an airspeed of600 miles per hour, if the current stabilizer angle of incidence 338 isnegative, the lower elevator position limit 376 may be −10 degrees andthe upper elevator position limit 374 may be +8 degrees. However, forthe same Mach and airspeed, if the current stabilizer angle of incidence338 is positive, the lower elevator position limit 376 may be −8 degreesand the upper elevator position limit 374 may be +10 degrees.

In some examples, the method may include selecting a value for the lowerelevator position limit 376 from one table of the second pair of lookuptables if the current stabilizer angle of incidence 338 is greater thanor equal to the threshold stabilizer angle, and selecting a value forthe lower elevator position limit 376 from the other table of the secondpair of lookup tables if the current stabilizer angle of incidence 338is below the threshold stabilizer angle of incidence 342. With referenceto FIG. 7, the method may also include selecting a value for an elevatorposition limit 374, 376 based on the status of the aircraft hydraulicsystem. For example, in the event of a hydraulic system failure, themethod may include selecting an elevator position limit 374, 376 that isless restrictive than the elevator position limit 374, 376 that may beselected in the absence of the hydraulic system failure. As indicatedabove, in the event of the hydraulic system failure, the elevatorposition limits (e.g., the deflection capability) of one or both of theelevators 360 may be increased as a means to maintain pitchmaneuverability of the aircraft 100.

Step 508 of the method 500 may include moving the elevator 360 to acommanded elevator position that is no greater than the elevatorposition limit 374, 376. As shown in the embodiment of FIG. 6, theflight control processor 402 may compute the upper elevator positionlimit 374 and the lower elevator position limit 376, and transmit theelevator position limits 374, 376 to the elevator actuator 378. Thepilot or autopilot may command the elevator actuators 378 to move theelevators 360 to a commanded elevator position which may be no greaterthan the elevator position limits 374, 376. In some embodiments, themethod may include determining whether the speed brakes 232 are deployedand, if so, positioning the stabilizer 330 at a stabilizer angle ofincidence 338 that results in a smaller download 312 or a larger upload310 than the respective download 312 or upload 310 generated by thestabilizer 330 at a stabilizer angle of incidence 338 for when the speedbrakes 232 are retracted. In this manner, the stabilizer 330 maycounteract a nose-up pitching moment that may be generated duringdeployment of the speed brakes 232.

FIG. 14 is a flowchart illustrating one or more operations that may beincluded in a method 600 of positioning an elevator 360 based upon apredetermined set of HTLA authority limits 370, 372. Step 602 of themethod 600 may include selecting a factor for increasing or decreasing apredetermined HTLA authority limit 370, 372 based on one or moreaircraft parameters 408. As indicated above, the HTLA authority limits370, 372 may decrease with an increase in Mach number and/or airspeed orother parameters (e.g., altitude) to compensate for any increase indynamic pressure with increasing Mach and/or airspeed. In someembodiments, the method may include selecting a reduction factor basedon the current stabilizer angle of incidence 338 as shown in FIG. 8 anddescribed above. As shown in lookup table, the reduction factor maydecrease with an increase in the current stabilizer angle of incidence338.

In some embodiments, the method may include calculating a predicted tailload 308 on the horizontal tail 306 with the elevator 360 at a givenHTLA authority limit 370, 372. The predicted tail load 308 may bedetermined based on one or more aircraft parameters including, but notlimited to, the current aircraft gross weight, the current aircraft CG114 location, the current aircraft pitch rate, and/or other aircraftparameters, as described above. The method may further includedetermining whether the predicted tail load 308 exceeds a load-carryingcapability of the horizontal tail 306 and/or fuselage 104. If thepredicted tail load 308 exceeds the load-carrying capability of thehorizontal tail 306 and/or fuselage 104, the method may includeselecting a reduction factor for the HTLA authority limit 370, 372 basedon the Mach and airspeed of the aircraft 100.

In some examples, the method may include selecting a reduction factorbased on Mach, airspeed, and current stabilizer angle of incidence 338as illustrated in FIG. 10. A reduction factor may be implemented by theflight control processor 402 to calculate the upper and lower elevatorposition limits 374, 376 based upon the upper and lower HTLA authoritylimits 370, 372 as shown in FIG. 9 and described above. Alternatively,the step of selecting a factor for the HTLA authority limit 370, 372 mayinclude selecting a reduction factor if speed brakes 232 are deployedand/or if the wing maneuvering-load-alleviation system 208 is activated,as illustrated in FIG. 11 and described above. In other embodiments, thestep of selecting a factor for the HTLA authority limit 370, 372 mayinclude selecting a factor to increase the HTLA authority limit 370, 372upon detection of a hydraulic system 380 (FIG. 1) failure of an elevatoractuator 378. For example, the flight control processor 402 may receivean indication of a failure of one or more of the hydraulic systems forthe elevator actuators 378. Upon receiving such indication of failure ofa hydraulic system, the flight control processor 402 may compute anincreased value for the upper elevator position limit 374 and/or thelower elevator position limit 376 in order to maintain pitchmaneuverability of the aircraft 100 during failure of the hydraulicsystem.

Step 604 of the method 600 may include computing an elevator positionlimit 374, 376 as a product of the HTLA authority limit 370, 372 and thefactor, as shown in FIGS. 8, 9, and 12. The flight control processor 402may compute an upper elevator position limit 374 and a lower elevatorposition limit 376 based upon upper and lower HTLA authority limits 370,372. As indicated above, the HTLA authority limits 370, 372 may bepredetermined based upon Mach number and/or airspeed of the aircraft100.

Step 606 of the method 600 may include moving the elevator 360 to acommanded elevator position that is no greater than the upper and lowerelevator position limit 374, 376 computed by the flight controlprocessor 402. As indicated above, the pilot or autopilot may commandthe elevator actuators 378 to move the elevators 360 to a commandedelevator position. The elevator actuators 378 may receive the commandfrom an elevator control input 406, and may pivotably move the elevator360 to the commanded elevator position, and may restrict movement of theelevator 360 to a position that is no greater than the upper and lowerelevator position limits 374, 376.

Additional modifications and improvements of the present disclosure maybe apparent to those of ordinary skill in the art. Thus, the particularcombination of parts described and illustrated herein is intended torepresent only certain embodiments of the present disclosure and is notintended to serve as limitations of alternative embodiments or deviceswithin the spirit and scope of the disclosure.

What is claimed is:
 1. A method of controlling an elevator of ahorizontal tail of an aircraft, comprising: selecting a factor forincreasing or decreasing a predetermined horizontal tail loadalleviation (HTLA) authority limit for an elevator based on at least oneaircraft parameter, the HTLA authority limit decreasing with an increasein Mach number and/or airspeed; computing an elevator position limit asa product of the HTLA authority limit and the factor; and moving theelevator to a commanded elevator position that is no greater than theelevator position limit.
 2. The method of claim 1, wherein the aircraftparameter is a current stabilizer angle of incidence of a stabilizer ofthe horizontal tail, the step of selecting a factor for the HTLAauthority limit comprises: selecting a reduction factor based on thecurrent stabilizer angle of incidence.
 3. The method of claim 2, whereinthe step of selecting the reduction factor includes: decreasing thereduction factor with an increase in the current stabilizer angle ofincidence.
 4. The method of claim 1, wherein the step of selecting afactor for the HTLA authority limit comprises: calculating a predictedtail load on the horizontal tail with the elevator at a given HTLAauthority limit; determining whether the predicted tail load exceeds aload-carrying capability of the horizontal tail; and selecting areduction factor for the HTLA authority limit based on the Mach numberand/or airspeed of the aircraft if the predicted tail load exceeds theload-carrying capability.
 5. The method of claim 4, wherein the step ofselecting the reduction factor for the HTLA authority limit comprises:selecting a reduction factor based on Mach, airspeed, and currentstabilizer angle of incidence.
 6. The method of claim 1, wherein thestep of selecting a factor for the HTLA authority limit comprises:selecting a reduction factor if speed brakes are deployed and/or if awing maneuvering-load-alleviation system is activated.
 7. The method ofclaim 1, wherein the step of selecting a factor for the HTLA authoritylimit comprises: selecting a factor to increase the HTLA authority limitupon detection of a hydraulic system failure of an elevator actuator. 8.The method of claim 1, further comprising: moving the elevator to anelevator position that is no greater than the elevator position limit.9. The method of claim 8, wherein moving the elevator comprises: movingthe elevator to the elevator position in response to an elevator commandinitiated by a pilot or by an autopilot.
 10. A system of controlling anelevator of a horizontal tail of an aircraft, comprising: a flightcontrol processor configured to select a factor for increasing ordecreasing a predetermined set of horizontal tail load alleviation(HTLA) authority limits for different elevator positions based on atleast one aircraft parameter, the predetermined set of HTLA authoritylimits decreasing with an increase in Mach and/or airspeed; the flightcontrol processor is configured to compute an elevator position limit asa product of the HTLA authority limit and the factor; and an elevatoractuator configured to move the elevator to a desired elevator positionthat is no greater than the elevator position limit.
 11. The system ofclaim 10, wherein: the aircraft parameter is a current stabilizer angleof incidence of a stabilizer of the horizontal tail; and the flightcontrol processor is configured to select a reduction factor based onthe current stabilizer angle of incidence.
 12. The system of claim 11,wherein: the flight control processor decreasing the reduction factorwith an increase in the current stabilizer angle of incidence.
 13. Thesystem of claim 10, wherein: the flight control processor is configuredto calculate a predicted tail load on the horizontal tail with theelevator at a given HTLA authority limit, and determine whether thepredicted tail load exceeds a load-carrying capability of the horizontaltail; and the flight control processor is configured to select areduction factor for the HTLA authority limit based on the Mach and/orairspeed of the aircraft if the predicted tail load exceeds theload-carrying capability.
 14. The system of claim 10, wherein: theflight control processor is configured to select a reduction factorbased on Mach, airspeed, and current stabilizer angle of incidence. 15.The system of claim 10, wherein: the flight control processor isconfigured to select a reduction factor if speed brakes are deployedand/or if a wing maneuvering-load-alleviation system is activated. 16.The system of claim 10, wherein: the flight control processor isconfigured to select a factor to increase the HTLA authority limit upondetection of a hydraulic system failure of the elevator actuator.
 17. Asystem for controlling an elevator of an aircraft, comprising: a flightcontrol processor configured to receive a stabilizer signalrepresentative of a current stabilizer angle of incidence of astabilizer having an elevator pivotably coupled to the stabilizer andselect an elevator position limit based on the stabilizer signal,wherein the selected elevator position limit is more restrictive if thecurrent stabilizer angle of incidence is equal to or above a thresholdstabilizer angle of incidence, and less restrictive if the currentstabilizer angle of incidence is below the threshold stabilizer angle ofincidence; and an elevator actuator configured to move the elevator toan elevator position that is no greater than the elevator positionlimit.
 18. The system of claim 17, wherein the threshold stabilizerangle of incidence corresponds to a value below which speed brakes ofthe aircraft are retracted.
 19. The system of claim 17, wherein theflight control processor is further configured to select, for a givenMach number and airspeed, an upper elevator position limit and a lowerelevator position limit having a magnitude different from a magnitude ofthe upper elevator position limit.
 20. The system of claim 17, whereinfor a given Mach number, airspeed, and current stabilizer angle ofincidence, the flight control processor is further configured to selectanother elevator position limit which is less restrictive than theselected elevator position limit responsive to an indication of ahydraulic system failure.